1. Field of the Invention
The present invention relates to systems and methods for dissolving a gas, such as oxygen, in a liquid, and still further to systems and methods for dissolving a gas into wastewater for use in a force-main sewer or gravity sewer application.
2. Background of the Related Art
Municipal sewage (which, as used hereinafter, includes wastewater) collection, conveyance and treatment systems (hereinafter referred to as “sewage treatment systems”) are extensive networks of interconnected pipeline infrastructure. Because of expansiveness and topography of municipal sewage treatment systems, sewage flows cannot always be conveyed by gravity. As a result, pumping sites are sometimes needed as part of the pipeline infrastructure to lift sewage flows over high points in the system or to the municipal treatment facilities.
A pumping site may be provided at a collection point of a project such as a housing development, an apartment complex development or a commercial office facility development. The pumping site is used to pump untreated sewage collected from the development through a “force-main” to a higher point where the untreated sewage can then flow via gravity to the municipal sewage treatment facility to be treated. Pumping sites may also be provided at low points in the pipeline infrastructure where gravity flow is no longer able to deliver the untreated sewage to the sewage treatment facility due to the topography. In most cases, the pumping site employs a collection sump, which receives and accumulates the untreated sewage from the upstream network of piping, and a mechanical pump which then pumps the sewage from the collection sump through the force-main to a higher point where the untreated sewage can flow by gravity to the sewage treatment facility to be treated. In a typical municipality, the sewage pipeline infrastructure of the sewage treatment system will contain numerous pumping sites to enable the flow of untreated sewage to reach the sewage treatment facility via both the force-main and gravity sewer pipelines.
Under anaerobic conditions, naturally present bacteria in a sewer system will convert sulfate compounds into reduced sulfur compounds, which are known to cause bad odors in sewers. One of these compounds, hydrogen sulfide, is known to form sulfuric acid when reacting with residual oxygen in the headspace of the sewer, which creates corrosion problems for steel and iron piping and other sewer components. By dissolving oxygen into the wastewater in a sewer system, the formation of an anaerobic condition is prevented, thereby reducing odor and controlling corrosion. Therefore, oxygentation of both force-main and gravity sewers can provide substantial benefits, and cost savings, when considering the operations and maintenance needs of these types of systems over their life-cycle.
Various systems are currently available for the pre-treatment of wastewater, including those that utilize oxygen for odor and corrosion control. For example, a generic pre-treatment system is described in U.S. Pat. No. 6,284,138 B1 to Hydro Flo, Inc. Further, a more specific application of superoxygenation for odor/corrosion control is disclosed in U.S. Pat. No. 7,566,397 B2 to Eco Oxygen Technologies, LLC.
However, the above-described systems have several disadvantages when considering force-main and gravity sewer oxygenation applications. One of the main disadvantages is the inability to achieve dissolved oxygen concentrations greater than 300-mg/L, which is desirable due to the large oxygen demand in these types of systems. This is attributed primarily to the above-described technologies only operating at pressures slightly higher than contained in the pipeline system. Because of this, the above-described systems typically required larger vessels, piping and footprint, and ultimately result in higher capital costs. Furthermore, the design of the above-described systems typically limits the ability to tightly control oxygen delivery based on changing site conditions. Often times, this results in the over-design of systems, making them less cost-effective due to the decreased efficiencies. Finally, the use of gas bubbles for oxygen dissolution by the above-described systems is generally a less efficient means of dissolving oxygen and has the potential to create operational issues in force-main sewer applications by increasing the amount and frequency of air release valve operation.
Many different systems and methods are available for dissolving gases in liquids and are highly dependent on the needed application. Those methods for dissolving oxygen into water are specifically referenced here. Most dissolved gas delivery methods—bubble diffusion, Venturi injection, U-tubes, and Speece cones, for example—are based on increasing the contact time or surface area of gas bubbles introduced into a bulk liquid to enhance mass transfer.
Most, if not all, of these earlier technologies require recovery systems for off-gases that do not dissolve into the fluid or allow loss of undissolved gases. For example, U.S. Pat. No. 5,979,363 to Shaar describes an aquaculture system that involves piping a food and oxygen slurry into a pond. U.S. Pat. No. 5,911,870 to Hough discloses a device for increasing the quantity of dissolved oxygen in water and employs an electrolytic cell to generate the oxygen. U.S. Pat. No. 5,904,851 to Taylor discloses a method for enriching water with oxygen that employs a Venturi-type injector to aspirate gas into a fluid, followed by mixing to increase dissolution. U.S. Pat. No. 5,885,467 to Zelenak discloses mixing a liquid with oxygen using a plurality of plates or trays over which the liquid flows gradually downward. U.S. Pat. No. 4,501,664 to Heil discloses a device for treating organic wastewater with dissolved oxygen that employs several process compartments. U.S. Pat. No. 5,766,484 to Petit describes a dissolved gas flotation system for treatment of wastewater wherein the relative location of inlet and outlet structures reportedly maximizes the effect of air bubbles in separating solids from the fluid. U.S. Pat. No. 5,647,977 to Arnaud describes a system for treating wastewater that includes aeration, mixing/flocculating, and contact media for removing suspended solids. U.S. Pat. No. 5,382,358 to Yeh discloses an apparatus for separation of suspended matter in a liquid by dissolved air flotation. And U.S. Pat. No. 3,932,282 to Ettelt discloses a dissolved air flotation system that includes a vertical flotation column designed with an aim of preventing bubble breakage.
Mazzei injectors (see, e.g., U.S. Pat. Nos. 5,674,312; 6,193,893; 6,730,214) use a rapid flow of water to draw gas into the fluid stream; mixing chambers may or may not be used to increase contact time between the gas bubbles and the fluid to increase dissolution. The system described in U.S. Pat. No. 6,530,895 to Keirn has a series of chambers under pressure that add gaseous oxygen to fluid; the pressure increase and the chambers in series are used to increase dissolution. U.S. Pat. No. 6,962,654 to Arnaud describes a system that uses a radially grooved ring to break a stream of fluid into smaller streams; gas is introduced into the streams and mixing is used to increase dissolution. Speece (see U.S. Pat. Nos. 3,643,403; 6,474,627; 6,485,003; 6,848,258) proposes the use of head pressure to introduce liquid under pressure into a conical chamber; the downward flow of the fluid is matched in velocity to the upward flow of gas bubbles to increase dissolution time. Littman et al. (U.S. Pat. No. 6,279,882) uses similar technology to Speece except that the upward flowing bubble size is decreased with a Shockwave. Roberts, Jr. et al. (U.S. Pat. No. 4,317,731) propose turbulent mixing in an upper chamber to mix gas with a bulk fluid; a quiescent lower chamber allows undissolved gas to rise back into the upper chamber for remixing.
Other U.S. patents describe various methods of increasing the contact time between gas bubbles in fluids, including U.S. Pat. No. 5,275,742 to Satchell; U.S. Pat. No. 5,451,349 to Kingsley; U.S. Pat. No. 5,865,995 to Nelson; U.S. Pat. No. 6,076,808 to Porter; U.S. Pat. No. 6,090,294 to Teran; U.S. Pat. No. 6,503,403 to Green; and U.S. Pat. No. 6,840,983 to McNulty. Spears, et al. (U.S. Pat. Nos. 7,294,278; 7,008,535) describe a method for varying the dissolved oxygen concentration in a liquid by varying the pressure from 14.7 to 3000 psi inside an oxygenation assembly. Patterson, et al. (U.S. Pat. No. 6,565,807) describe a method for maintaining, adjusting, or otherwise controlling the levels of oxygen dissolved in blood (e.g., pO2) by controlling the flow rates or by providing controlled amounts of the blood or oxygen gas.
These conventional systems for dissolving gases in liquids, and in particular conventional dissolved oxygen delivery systems are based on dissolving bubbles into stationary or flowing water and are greatly limited in the range of dissolved oxygen concentration that can be attained and controlled. These conventional systems are also limited to nearly steady-state use, and cannot quickly adjust dissolved oxygen concentrations to optimize water treatment. Bubble-based technology is limited to much lower dissolved oxygen concentration in the water being treated because of lower pressure and less-efficient gas transfer.
Accordingly, there is a need for systems and methods for optimizing the dissolution of a gas into a liquid, and in particular for systems that can be cost-effectively applied to force-main and gravity sewers for odor and corrosion control by adjusting dissolved oxygen concentrations to levels that are optimal for maintaining aerobic conditions. The systems and methods described in this disclosure meet this need.